Abstract

Do you realize that you are constantly bombarded by particles? You do not feel them, you cannot see, hear, or smell them, but they are always there! These particles — collectively called background radiation — might even travel through you without ever interacting with the molecules in your body. In this science project, you will build your own cloud chamber to prove the existence of background radiation. You will then use your cloud chamber to determine if the background radiation in your environment appears to be random.

Objective

In this physics science project, the student will create a cloud chamber to visualize background
radiation and determine if the radiation particles appear randomly.

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Introduction

Are you tricking me? Particles going through my body without my feeling it? Something zooming past my ear without any audible sound, flying under my nose without a trace of smell? What is this mysterious thing called background radiation? Where does it come from? Is it everywhere? Is it safe?

A little review before we tackle these questions. Atoms are extremely small particles that we, and everything around us, are made of. As shown in Figure 1, atoms consist of a tiny area called the nucleus and a cloud of electrons that surrounds the nucleus. Electrons are negatively charged, and they balance the positive charge of the nucleus, which results in the atom being electrically neutral.

Figure 1.
Representation of an atom with its nucleus and an electron cloud around it. Note that in this drawing, the nucleus shown is disproportionately large.

Atoms can both gain and lose electrons. The process of gaining or losing an electron is called ionizing. An atom that has lost or gained one or more electrons is called an ion. How this relates to background radiation will be explained, but first let us explore the term "radiation."

Radiation is energy that travels through space either as high-speed particles or as waves. We encounter radiation around us all the time. For example, light bulbs radiate visible light; microwaves radiate waves that cook our food; an x-ray machine emits (or releases) x-rays. All these are examples of electromagnetic radiation. Watch the video to learn more intriguing facts about electromagnetic radiation, insights that will help you understand this science project.

This video gives an introduction to light and electromagnetic radiation.

We can see, feel, or hear other types of radiated particles or waves, but we cannot actually sense most radiation. Background radiation, unsurprisingly, is made up of radiated particles. Now that you know a little about radiation, can you come up with a definition of background radiation?

Background radiation is the term used to describe all ionizing radiation that we encounter in our environment on Earth. Did you notice the term ionizing in the definition, the term we explained before? In other words, background radiation is made up of particles or waves in our environment that are so energetic that they ionize the matter around them, making the matter gain or lose an electron.

So what are these particles, so energetic that they cause ionizing radiation? We will not go into great detail here, as it would involve the fascinating subject of elementary particles. But maybe you have heard of some particles involved, such as photons, electrons, muons, protons, neutrons, alpha particles, or neutrinos. All these are tiny particles that, when colliding with an atom, can cause the atom to lose or gain electrons. Have you heard of any others? Do not worry if all these names sound new. This Introduction will provide you with all the information necessary to understand your observations in this science project as well as links (in the Bibliography) to explore these concepts further.

You now know that background radiation is due to energetic particles found all around us, but where do those particles come from? There are two primary sources of background radiation: cosmic (from space beyond Earth's atmosphere) and from the materials on Earth itself. Cosmic radiation is the radiation that Earth is constantly bombarded with from outside the solar system, in addition to high-energy ion radiation emitted by the Sun during solar flares. This radiation consists primarily of particles with extremely high energy. They interact in Earth's atmosphere to create secondary, lower-energy background radiation, some of which travels to Earth's surface. On Earth, radioactive material (material that emits ionizing particles) is present all around us. In the soil, radioactive materials like potassium, uranium, thorium, and radon are present throughout the world in varying concentrations. Some radioactive materials (such as carbon 14) are present in any living being as part of the building blocks of life. Human beings have learned to use radioactive materials in a number of applications (such as in nuclear power plants and in some types of medical treatments). Radioactive materials send out particles randomly in all directions.

Knowing its origin, do you expect background radiation to come mainly from the sky (radiation particles coming down from Earth's atmosphere), from the surface (radioactive material released from particles in the soil), or from all directions equally? Figure 2 might give you some hints.

Figure 2.
Secondary radiation formed in the atmosphere as ionizing cosmic particles collides with atoms in the atmosphere (part A) and background radiation formed from radioactive material on earth (part B). (Note that objects in the graphics are drawn disproportionate to their real size.)

In this science project, you will categorize the particles you see into three groups, based on their position relative to Earth: vertical (from the sky down or from Earth straight up); more or less horizontal; and at an angle. Do you expect one group to be more prevalent than another?

If background radiation cannot be seen, felt, or heard, how do we know it is there? How can we study it? Scientists build cloud chambers, also known as Wilson chambers, to study background radiation particles. Much as you would study the wind by looking at the movement of leaves, scientists study background radiation by looking at tracks in cloud chambers left by the passage of radiation particles. You cannot see the wind, but you can study its presence, direction, and strength by seeing how the leaves on trees move. In this science project, you will not "see" the background radiation particles, but you will show that they exist and study them by observing the tracks they create in your cloud chamber.

Before we explore how a cloud chamber works, let us review some theory about vapor and condensation. When we heat a liquid, it converts into a vapor; when we cool a vapor, it condenses into a liquid. A saturated vapor is a vapor ready to condense. Tiny solid or liquid particles (like dust or tiny droplets) must be present to produce the transition from vapor to liquid. Those particles are called condensation nuclei. A supersaturated vapor is ready to condense into a liquid, but it lacks the condensation nuclei to make that possible.

So where is the link with a cloud chamber? A cloud chamber is nothing more than a sealed environment containing a supersaturated vapor of water or alcohol. When ionizing particles from background radiation (like muons or electrons) travel through the chamber, they collide with the surrounding molecules, creating ions (charged atoms) along the way. These ions leave a trail of condensation nuclei, giving the supersaturated vapor something on which to condense. At this stage, tiny condensation droplets form on ions left by background radiation particles. A bright light will bounce off the droplets and there you have it — a visible track!

In short, you see droplets of alcohol form on ion trails left by a particle of background radiation passing through the cloud chamber.

Did you note that only muons and electrons were listed in the explanation? Why? Would the other background radiation particles not provide a track in the cloud chamber?

Different particles have different capacities to penetrate material and interact with the atoms in the chamber. Some are not able to penetrate through a layer of plastic (like alpha particles) and others can fly through Earth without any interaction (like neutrinos). The muon and some high-energy electrons can penetrate easily through the chamber's plastic walls and interact enough with material to show tracks in the chamber. In short, in the cloud chamber you are about to build, you will most probably see only muons, and maybe some electrons. Any idea where these originate? Note: Although you might like to see what happens in your chamber when you place it near a radioactive source, we do not advise that you spend time in the vicinity of the source (unless it is specifically marked low dose — for student experiments).

In this physics science project, you will build a cloud chamber and use it to study the background radiation present in your environment and see if any patterns in the radiation are present.

Terms and Concepts

Atoms

Electrons

Ionizing

Ion

Radiation

Electromagnetic radiation

Background radiation

Ionizing radiation

Cosmic radiation

Radioactive material

Cloud chamber or Wilson chamber

Vapor

Condensation

Saturated

Condensation nuclei

Supersaturated

Questions

Knowing the origin of background radiation, do you expect background radiation to vary with altitude or to be concentrated at specific places in the world?

As background radiation is not visible, how can we observe and study it? Hint: Think about what the trails in a cloud chamber are and how they are made.

How are background radiation and ionizing radiation related?

Is there a difference between radiated particles and radioactive particles?

What are the necessary components of a cloud chamber and what circumstances are essential for a cloud chamber to work?

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Materials and Equipment

Clear plastic cup with lid, such as a deli cup (approximately 8 cm tall and 9 cm in diameter is a size that works well, but similar sizes also work). Adjust indicated quantities if using a larger cup.

Thick felt (15 x 15 cm). Adjust size of square if using a much larger cup.

Fine-point marker or pen

Scissors

Small amount of Plasticine® or modeling clay (size of a ping pong ball — adjust quantity if using a larger cup). It can be ordered online from
Amazon.com

Black construction paper (1 sheet)

Glue or tape

Room that can be made dark

Protective gloves (to handle dry ice)

Safety glasses or goggles

Block of dry ice (about 2 lb.). Can be found in well-equipped grocery stores, most often stored in a separate freezer located next to the regular ice. Bring a (small) isolating box or bag to store the dry ice on your way home. It keeps well in a closed cooler or freezer. When transporting dry ice remember to put it in a container with a lid that shuts to prevent evaporation, but do not seal the dry ice in an airtight container because this can cause a buildup of gas that may result in an explosion. Always
wear protective gloves when handling dry ice. Bring your gloves to the store.

Baking tray. The tray must be larger than the clear plastic cup.

Hammer

Isopropyl alcohol 91% (40 ml). Available at well-equipped drug stores or order it online from
Amazon.com

Eye dropper

Bright-beam LED flashlight or headlamp

Lab notebook

Helper

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Experimental Procedure

Prepare the Cloud Chamber

In the first part of this science project, you will prepare your cloud chamber so you can investigate background radiation around you.

Cut a piece of felt to fit in the bottom of your cup. The felt will hold the alcohol.

Place the cup on the square of felt (bottom down, lid side up).

Use a fine-point marker or pen to trace around the bottom of the cup on the felt.

Cut out the circular piece you traced from the larger square of felt.

Place the felt on the inside bottom of the plastic cup.

Mold and use the Plasticine or modeling clay to hold the felt in place at the bottom of the cup.

Roll the clay (or Plasticine) into a long, skinny (about .5 cm in diameter) rope long enough to place around the circumference of the inside bottom of the cup.

Place the clay roll around the border of the felt in the cup and push it in so the clay holds the felt in place, as shown in Figure 3.

Test if the felt stays put by turning your cup upside down.

Figure 3.
The first steps in preparing a cloud chamber should result in a cup whose inside bottom is covered with felt and Plasticine or clay holding the felt in place. In this example, the felt is black and the Plasticine is green, but it does not matter what colors you use.

Use the black construction paper to make a black background that will fit inside the lid of the cup. When the cloud chamber is complete, the cup will be turned upside down, so that the black background will be on the bottom. This is because the particle tracks will be more visible against a black background.

Cut a circle the size of the inside of the lid of the cup from the black construction paper.

Cover the inside of the lid with the circle of black construction paper. Use glue or tape to make the construction paper adhere to it.

Close your cup, put it upside down (resting on the rim) as shown in Figure 4, and there is your cloud chamber! For an explanation of how to use it, see the next section.

Figure 4.
Picture of a cloud chamber ready to use, with the cup turned upside down, the felt held in place by the clay on the inside bottom of the cup, and the circle of black construction paper fitting inside the lid of the cup.

Observing Background Radiation

Now that you have prepared your cloud chamber, you can fill it with supersaturated vapor and use it to detect background radiation in your environment. Your observation should last about 10 minutes; it can be longer if your chamber still produces nice clear tracks. Allow for about an hour to complete the project, as it might take a while before you have the right supersaturated vapor environment and tracks appear in the chamber, enabling you to begin your observations.

In your lab notebook, make a data table similar to Table 1.

Time Interval

Horizontal Track

Vertical Track

Slant Track

Total Number of Tracks(Regardless of Direction)

Total

Table 1. In your lab notebook, create a data table similar to this one. In this data table you will record how many different types of tracks you see in your cloud chamber over time.

You will be recording how many tracks you see, and the angles at which they are visible in this data table. From your observations, you will determine if the observed radiation appears random in direction.

Classify the tracks you see depending on their inclination with respect to the vertical and horizontal planes:

Horizontal tracks: tracks with a relatively small inclination with respect to the horizontal plane.

Vertical tracks: tracks with a relatively small inclination with respect to the vertical axis.

Slant tracks: tracks that are neither vertical nor horizontal.
Note: You will not be able to measure the angle of inclination exactly. Your best estimate will do fine for this project. Figure 5 shows how to classify tracks.

Figure 5.
How to classify tracks in three groups, depending on their inclination with respect to the vertical and horizontal planes. The cones indicate the area in which to classify a track as vertical (small inner cone), slant (wide cone, but not in the smallest cone), or horizontal (outside the cones).

Take a moment to think about what you expect to see.

Do you expect tracks to appear randomly in their direction or take a specific incident inclination?

Do you expect tracks to appear randomly in time?

Choose an environment that can be made dark (it can be inside or outside) and away from any heat source. Along with your cloud chamber, bring all the materials listed below "Room that can be made dark" in the materials list with you.

Create a bed of small pieces of dry ice for your cloud chamber.

Put on the protective gloves and glasses or goggles to handle the dry ice.

Place the dry ice on the baking tray and crush it into small pieces with your hammer. Smaller pieces create a more even contact with the lid of your cup and avoid relatively cooler and warmer spots in the cloud chamber during the experiment.

You can take the gloves off, but be aware that the baking tray will get cold — really cold. Avoid touching it with your bare skin.

Soak your felt with the isopropyl alcohol using the eye dropper.

Though you should not make the felt dripping wet, it is important to thoroughly soak it.

Trap alcohol vapor inside the cold chamber by quickly snapping on the lid of your cup.

Place the cup upside down on the dry ice. Your setup should now look similar to the one in Figure 6.

Figure 6.
In your setup of the cloud chamber, the cup should be positioned upside down on the layer of crushed dry ice you made on the baking tray .

Warm the cup so the isopropyl alcohol will evaporate and create a fine vapor in the chamber

Place the palm of your hand on top of the inverted cup to warm the alcohol-soaked felt.

Tip: A gradual evaporation of the alcohol works best, so do not put something warmer than your hand on the cloud chamber.

Kill the lights and light up the cloud chamber with a flashlight. Start looking for tracks, but keep in mind that it can take from a few minutes up to 20 minutes to see any tracks, so you may need to be patient. Once you start seeing tracks, move on to the next step, where you will start recording your observations.

The creation of alcohol vapor is necessary to see the tracks. You will not see the alcohol vapor immediately — it will only be apparent when it condenses into microscopic droplets, forming a mist. Depending on the size of your chamber and the specifics of your environment, it can take a few minutes up to 20 minutes to see the thin fog of alcohol vapor, so be patient.

Although it may take a few minutes for the alcohol vapor to form, you can start trying to see tracks by holding your flashlight at a 45-degree angle with respect to the baking tray, as shown In Figure 7 on the left.

As shown in Figure 7 on the right, experiment holding the flashlight at the 2 o'clock position if you are observing from the 9 o'clock position. You might have to tinker around with these suggested positions. The idea is to make the light reflect off droplets, but eliminate any glare. Additionally, sometimes ideal conditions for the supersaturation are met only in particular areas of the cloud chamber. The best place to observe tracks is where you see the thin mist, which may take up to 20 minutes to appear.

Figure 7.
These drawings illustrate the recommended position of the observer and the flashlight with respect to the cloud chamber to achieve good observations.

It is advised (but not obligatory) to keep your hand on top of the cloud chamber to give it just a bit of heat.

Once you start seeing tracks, move on to step 10 to start recording your observations.

If you do not see any tracks within 30 minutes, you might not have the right conditions in your cloud chamber. Before you start all over again, evaluate the following questions and see what you could do differently:

Was the felt well soaked with alcohol when you started, without excess alcohol dripping off the sides?

Is the cloud chamber nicely sealed or can vapor escape? If vapor does escape, seal the hole and try again.

Is there enough dry ice to keep the bottom of your chamber cool? Figure 7 can help you evaluate. Note that there is a layer of dry ice under the chamber (between the chamber and the baking tray) as well as around the edges of the chamber (visible in Figure 7).

Did you warm the top of the chamber with the palm of your hand? Faster warming with a much warmer heat source or not warming enough will result in a failing cloud chamber.

Is your flashlight strong enough to illuminate the chamber?

Did you try positioning yourself and the light source at various angles?

If you tried to tweak all of these variables and are still having problems seeing tracks, start over from step 5.

Once you observe tracks, ask your helper to assist you in taking notes of what you observe.

Tell your helper whether the track is more or less horizontal, more or less vertical, or slant (more or less at 45 degrees) with respect to the baking tray.

The helper should note the number of tracks you observe by tallying them in the appropriate box in the data table (similar to Table 1) in your lab notebook.

Stop observing when tracks become faint. This will probably happen after roughly 10 minutes.

Turn on the light and take a breather — you have been focusing for quite a while!

In your lab notebook, record some further general-impression notes on your observations:

Would you say the tracks came randomly in time (for example, at 1 second, then 3 seconds, then 10 seconds, etc.) or at a regular time interval (for example, every 5 seconds).

Were some/most/all tracks straight tracks? Or curved tracks or spirals? Did you observe other configurations?

Was there a variation in thickness of tracks? Were some clear and others faint?

Any other observations that caught your attention?

Clean up or prepare for another observation if you need to or like to repeat this experiment.

You can leave the dry ice to evaporate in warmer air or dispose of it in the sink. But put on the protective gloves to hold the baking tray, as it will be cold.

It is advised to observe tracks at least three times. You could choose to change the time of day to see if background radiation is different at different times of day. Keep in mind that, to make conclusions about a trend, you might need to make several more observations.

Finalize the observations and analyze the results.

In your data table, calculate the total number of tracks and write this down in the far right column of your data table.

What does your data tell you? Is there a pattern or consistency in the angles at which you see the tracks? Do the observations support or contradict your knowledge of background radiation? Do you have enough data to draw conclusions?

After some more background study, you would be able to conclude the tracks you see in the chamber are most probably muons with maybe an occasional electron. Other background radiation particles that reach the surface of the earth will either not penetrate trough the plastic of the
deli cup (e.g. alpha particles) or fly trough it without ever interacting with the vapor in the chamber (e.g. neutrinos).

This conclusion may not hold if you do your experiment near a strong radioactive source — which we do not recommend for safety reasons.

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Variations

You can repeat this science project at a higher or lower altitude (for instance, at sea level and/or on a hill or mountaintop with an elevation of 1,000 feet or more). Does altitude affect how much background radiation you observe?

What other factors cause certain locations to have more background radiation than others? Research background radiation a little more in-depth, finding out where background radiation exists more than in other places. What factors are important? Can you safely test any of these factors using your cloud chamber?

See if observations inside your home or outside your home or in different rooms of your home make a difference. If you have a basement or multiple floors in your house, see if your observations differ on different floors. Do you expect to see a difference? (Think about the origin of the radiation you observe, the type of particles and how well they penetration material).

Do you believe some materials (like plastic, cloth, paper, aluminum foil, cardboard, or wood) can block the background radiation you detect in your chamber? Test it by covering part of your cloud chamber with different materials. Be sure to leave an opening in the material big enough for you to watch the chamber and to shine a flashlight through it.

Ask an Expert

The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

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